Q.A toroid of narrow radius 10 cm has 1000 turns of wire. For a magnetic field of $5\times10^{-2}$ T along its axis, how much current is required to be passed through the wire?

A toroid is a solenoid of finite length bent around and closed into a hollow, ring- or tube-shaped structure, with no open ends. Its field lines form concentric circles running around its own central circular axis, entirely confined within the winding (unlike a solenoid, which leaks some field near its open ends). Applying Ampere's law to a circular Amperian loop of radius R running along this central axis, threading through all N turns of the winding, gives $B(2\pi R)=\mu_0(iN)$, so $B=\dfrac{\mu_0iN}{2\pi R}$. …